Cut the oranges and lemons; remove the pips, wrap in a muslin bag and set aside. Put the oranges and lemons through a mincer using coarse blades. Leave for 24 hours, covered with 6 pints of water.
Place the oranges, lemons, water and wrapped pips in a large pot. Heat, covered, over medium heat for 1 hour.
Add the sugar and bring to the boil until the setting point is reached, about 45 minutes. See footnote below.
After it has set, allow to settle for 15 minutes; pour into dry, clean, warmed jars. Immediately place a waxed disc on the surface and cover the jar with a cellophane covering or air tight lid. Store in a cool, dry and preferably dark cupboard.
In order to check for the setting point, place a teaspoonful of marmalade onto a chilled plate. Allow to cool for a moment and the push it with your finger. If a crinkly skin has formed, then it is set. If it hasn't set, boil again for another 5 minutes and retest.